Clean up MIPS ISA macros

[Richard Sandiford]

David Daney <ddaney@avtrex.com> writes:
> Perhaps cross builds should default to -Werror so that bootstrap 
> breaking problems are caught.

Yeah, that's a good point.  The problem with using -Werror for
cross-compilers is that you need to use a recent version of gcc as the
build compiler, otherwise you might get different warnings.  E.g. I know
my system gcc (3.4) can't compile the sources with -Werror because it
reports more warnings.

However, after introducing two separate warnings in the same day,
this is clearly something I need to do.  So FWIW, I've modified my build
scripts to bootstrap a native gcc first, and then use that gcc to build
the cross toolchain, configuring the latter with --enable-werror-always.

Richard